Add contract to `ConfigEntry#getString(String)` for non-null-argument

The contract basically says that if you provide a
non-null argument to the method, it cannot return
a null value.

Core/src/main/java/com/songoda/core/configuration/songoda/ConfigEntry.java

@@ -123,6 +123,7 @@ public class ConfigEntry {
         return value != null ? value.toString() : null;
     }
 
+    @Contract("!null -> !null")
     public @Nullable String getString(String fallbackValue) {
         Object value = get();